The main cause for derailing of an unpowered rail transport unit is poor maintenance resulting in flattened wheels.
When an unpowered rail transport unit brakes, its wheels will often block when the car is still in motion. These blocked wheels will slide over the rails until the train comes to a full stop. As both the rails as well as the wheels are made out of metal, this sliding will cause the wheel to flatten on the spot where it was sliding over the rails. The result of this ‘sliding and flattening’ will be a wheel that is not perfectly round anymore.
A flattened wheel in a bad shape will not turn round anymore and will cause vibrations and have the risk to derail because of its ‘non-adapted’ form on the rails. This is why every unpowered rail transport unit wheel has to be grinded and sharpened every 100,000 km and has to be changed together with the complete axle every 1,000,000 km, because of the fact that the wheelset, this means the axle together with the 2 wheels are always made out of 1 single piece of metal.
In practice, it appears to be very difficult to register the exact mileage of the rail transport units, this because of the lack of any power supply, the extreme working conditions such as shocks, vibrations, all weather conditions, extreme temperatures, etc. and also because of the enormous logistical burden to manually keep track of this kind of data.
Even when a company would be able to calculate the traveled mileage of its unpowered rail transport units, it would be confronted with a second and even bigger problem: the differences in the track gauge in several countries or areas in the railway grid. The track gauge in Western Europe will not always be the same as the track gauge in Eastern Europe or Asia. When transporting goods by rail towards countries with a different track width, a company has 2 options:    1. transhipment of the goods to other unpowered rail transport units that are adapted to the changed gauge of the tracks; or    2. to adapt the own railcars by changing the axles.
It often happens that the train operators change the axles on unpowered rail transport units on their own initiative to prevent too many delays on the total transport. The owners of the rail transport units and or companies that lease the unpowered rail transport units are often not informed and not aware of these actions. They also often don't know which axles are being put under their wagons, and in practice often used axles are being mounted.
Due to these actions, it becomes very difficult for unpowered rail transport unit owners to keep track with the real mileage of the axles on their unpowered rail transport units.
At present, companies make rough calculations based on the dispatch planning of each of their unpowered rail transport units to have an estimate of traveled miles per transport unit, but have no clue at all about the exact mileage per axle of the wheelset.
This method however results in very large errors, but it is better than not having any idea at all.
Another method to obtain control of the need of maintenance of the wheels on an axle of an unpowered rail transport unit is to use of a time parameter instead of taking into account the mileage in case of transportation of high risk or dangerous cargo. With high risk unpowered rail transport units for instance, a monthly check is done.
This method however is not accurate at all.
Still another possibility is to mount a mechanical mileage counter on the axles.
In U.S. Pat. No. 5,433,111 for instance, an apparatus for detecting defective conditions associated with a set of railway vehicle wheels and with a rail track upon which a given railway vehicle travels comprises a rotation measurement unit for generating data indicative of motion along a vertical axis relative to the rail track.
Another comparable system is disclosed in WO 2008/079456, wherein an odometer system of the type which measures distance traversed by a railway vehicle based on a number of wheel rotations is described. The system comprises a control unit that is coupled to receive vehicle position information from a vehicle position device and one or more signals corresponding to a number of wheel rotations. The control unit is programmable to determine distance traveled by the vehicle based on position information acquired at different times. The distance information is used to provide a measure of distance traveled by the wheel during rotation.
In DE 10 2010 027 490, a system is disclosed for monitoring wear and tear of rail bound goods or passenger transport unit in a wagon. This system comprises an electronic sensor for determining a distance traveled by the car on the basis of the measured speed of the sensor. The sensor is integrated in the wheel bearing of the wagon. A speed sensor is equipped for measuring the rotating speed of the wheel bearing. A sensor electronics is connected to the speed sensor and is equipped for determining a distance traveled by the wagon based on the rotating speed measured by the speed sensor. The determined distance of the sensor electronics is compared with a certain independent distance and when there is a differential distance, slip is detected between the wheel bearing and the traffic rail. This independent distance is determined by a second satellite based measurement system.
A first big disadvantage of the systems having a mileage counter on the axles is that it is not detectable when axles are removed from beneath the wagon. It is consequently not at all possible to know the correct mileage of these axles.
Another disadvantage thereof is the need of human interaction on regular basis to check the counters of all axles and to manually put this into a database. Knowing that operators always own/operate several thousands of unpowered rail transport units and knowing that unpowered rail transport units stay out in the grid for 4 years before having to come in for inspection, this implies a gigantic manual operation that is commercially not feasible in reality.
A further monitoring system for monitoring the axles present beneath an unpowered transport unit is known from US2011/231039. It describes an embodiment which comprises a sensor device mounted on each shaft or vehicle axis of a rail vehicle. A telematics unit can be mounted on the rail vehicle and receives signals concerning start of movement, mileage, velocity, rotational direction, blockage of the wheels, brake activity etc. from each of these sensor devices via a wireless radio link. It is further acknowledged that as the individual vehicle axels can be individually replaced, they can have mileages different from the rail vehicle. The telematics unit can also examine the position of the rail vehicle by means of GPS. It is further disclosed that the sensor devices could store a unique identifier for the shaft on which they are mounted. However the sensor device requires, next to the suitable sensor systems for determining the mileage of the axle, also a radio module suitable for setting up wireless communication channel with the telematics unit in order to transmit this determined mileage from the sensor device to the telematics unit. It is clear that such a setup of the sensor device necessitates an on-board power supply in the form of battery, which leads to a the need for providing a counterweight and additional complexity as the battery needs to be replaced in a timely manner. Additionally, as the mileage of the axles is determined in the sensor device itself, the monitoring system is only able to cope with axles which are provided with such a sensor device. Especially when one or more of the wheelsets of a rail vehicle are for example replaced with wheelsets that are not equipped with such a sensor device, the system will not be able to reliably track the mileage during which such unidentified axles have been used. Additionally, replacement of one axle with another axle according to this system is not considered a dangerous situation, while, especially in the case of for example providing unidentified used axles to a rail cart comprising hazardous goods in practice does pose a risk, which would go undetected and without issuing an alert.
It is known from EP1382507 to provide a passive transponder to the axle of a wheelset of a rail vehicle comprising a unique identifier for the wheelset, which can be detected by means of a handheld detector within a distance of 2 to 4 cm. However such a passive transponder does not comprise suitable sensor systems do determine the mileage of the axles of the wheelsets.
There therefore exists a need for a monitoring system providing in a more accurate, robust and a simpler way the determination of the traveled mileage of the axles of unpowered rail transport units, which monitoring system furthermore allows to more reliably detect when one or more axles are removed from beneath the unpowered transport unit and/or to detect replacement of one or more axles, especially replacement by one or more unidentified axles.